Ink delivery method for electrostatic printing



June 10, 1969 E. w. PITTENGER ETAI. 3,448,687

INK DELIVERY METHOD FOR ELECTROSTATIC PRINTING Original Filed May 6, 1965 o) FIG I INVENTOR JAMES W. EDWARDS BY EVAN W. PITTENGER $1 AT TORNEY June 10-, 1969 w, lTr-Ewsgn ET AL 3,448,687

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INK DELIVERY METHOD FOR ELECTROSTATIC PRINTING Original Filed May a, 1965 Sheet 5 of 5 (O n will a E I f 0 2 i I I N Maw w INVENTOR JAMES W. EDWARDS BY EVAN W. PITTENGER ATTORNEY June 10, 1969 E. w. PITTENGER ET AL INK DELIVERY METHOD FOR ELECTROSTATIC PRINTING Sheet of5 Original Filed May 6, 1965 N 2. o. a Q? ND mo 1 v0 INVENTOR JAMES W. EDWARDS BY EVAN W. PITTENGER ATTORNEY June 10, 1969 E. w. PITTENGER ET AL 3,448,637

INK DELIVERY METHOD FOR ELECTROSTATIC PRINTING Original Filed May 6, 1965 Sheet 5 of 5 FIG. IQ

INVENTOR JAMES W. EDWARDS BY EVAN W; PITTENGER ATTORNEY United States Patent 3,448,687 INK DELIVERY METHOD FOR ELECTROSTATIC PRINTING Evan W. Pittenger, St. Louis, and James W. Edwards, Creve Coeur, Mo., assignors to Monsanto Company, St. Louis, Mo., a corporation of Delaware Original application May 6, 1965, Ser. No. 453,706, now

Patent No. 3,339,483, dated Sept. 5, 1967. Divided and a this application June 26, 1967, Ser. No. 669,993

Int. Cl. B413 1/46; B41m 1/12, N00

US. Cl. 101-426 7 Claims ABSTRACT OF THE DISCLOSURE This is a division of application Ser. No. 453,706, filed May 6, 1965, now Patent No. 3,339,483, granted Sept. 5, 1967.

This invention relates in general to certain new and useful improvements in electrostatic printing and more particularly, to a mechanism for creating triboelectric charges and delivery 'of ink particles.

In the presently available electrostatic printing devices, particularly in electrostatic screen process printing 'apparatus and in electrostatic coating apparatus, the article which is to be printed or coated is supported in spaced relationship to a discharge electrode and a receiving electrode. A substantial potential difference is maintained between these electrodes in order to create an electrostatic field. In most of the commercially available electrostatic printing devices, the inks which are employed are generally formed of a resinous base material such as natural rosin or various synthetic polymers and are designed to carry a suitable coloring material. These types of particles are extremely small, in the area of 2 to 20 microns and are capable of triboelectric charging.

Due to the fact that the ink particles are relatively small and extremely light in weight, they are readily compressible and moreover, very difficult to control. These small ink particles also have a tendency to cling together in agglomerates and, due to the resinous nature of the particles, become permanently bonded together when subjected to moderate compression. Inasmuch as they are capable of being triboelectrically charged, they are always aifected by static charges. In the actual electrostatic printing operation, it is often difficult to maintain a substantially uniform field strength. Consequently, the field strength is always relatively higher over and above partially defined edge portions and isolated prominences rather than over the relatively broad expansive connecting surfaces of the article. The influences of the field strength has substantial effects on these lightweight ink particles and thereby tends to deposit heavier coatings on such edge portions than on the body portions due to this lack of balance in distribution. Moreover, due to the fact that agglomerates of various sizes exist, the individual ink particles and masses acquire widely different amounts of charge during the triboelectric charging process, which also is a cause of irregular print coverage during the screen printing operation.

Patented June 10, 1969 It is known that the even distribution and flow of ink particles materially affects the efficiency of the printing operation and of the line definition obtained. Problems of metering a selected quantity of ink particles, spreading the ink particles on a carrier, creating triboelectric charges on the particles and transporting the charged particles to an electrically charged screen are all interconnected and atfect the quality of the electrostatic image produced.

In the devices of the prior art, distribution of ink particles on an ink carrier was uneven due tothe fact that there was no elfective device for simultaneously metering and delivering a charge of ink. Subsequent print from these devices suffered areas of reduced color density, as a result of uneven ink distribution on the ink carrier. These areas of reduced color density have the outline of the previously printed pattern, often referred to as ghosting or memory effects and materially reduces the asthetic appearance of the print.

The prior art has attempted to solve the problem of even distribution of coating particles by promoting a more uniform field strength over all of the surface portions of the article to be coated. One moderately effective method 'of obtaining a more uniform field strength is described in US. Letters Patent No. 2,428,991, through the use of specially biased electrodes. However, devices of this last named type are expensive, and are not capable of being modified so that the printing devices of this type are limited in their degree of utility.

It is, therefore, the primary object of the present invention to provide an apparatus for use with an electrostatic printing system which is capable of metering a uniform supply of electroscopic ink to the screen of such electrostatic printing system.

It is another object of the present invention to provide an apparatus of the type stated which is capable of maintaining uniform distribution and continuous delivery of ink particles to the screen of an electrostatic printing system.

' It is also an object of the present invention to provide an apparatus of the type stated which does not produce memory or ghosting effects during electrostatic printing operations.

It is a further object of the present invention to provide an apparatus of the type stated which is capable of simultaneously creating even distribution of ink particles and triboelectrically charging such particles.

It is another salient object of the present invention to provide an apparatus of the type stated which is adaptable for use in mass-production methods of electrostatic printing and which does not require the employment of specially designed electrodes or devices for continually altering the electrostatic field.

It is an additional object of the present invention to provide an apparatus of the type stated which is capable of accommodating a wide variety of types, colors and sizes of inks to be used in the electrostatic printing system.

With the above and other objects in view, our invention resides in the novel features of form, construction, arrangement and combination of parts presently described and pointed out in the claims.

In the accompanying drawings:

FIGURE 1 is a front elevational view of an ink delivery system constructed in accordance with and embodying the present invention and showing a slide tray forming part thereof in an extended position;

FIGURE 2 is a rear elevational view of the ink delivery system of FIGURE 1;

FIGURE 3 is a top plan view of the ink delivery system of FIGURE 1;

3 FIGURE 4 is a left end elevational view of the ink delivery system of FIGURE 1;

FIGURE is a right end elevational view of the ink delivery system of FIGURE 1;

FIGURE 6 is a vertical sectional view taken along General description Generally speaking, the present invention comprises an apparatus which is designed for use in mass-production electrostatic printing systems of the type described in copending application Ser. No. 463,109 filed June 11, 1965.

An electrostatic printing system of the type with which the present apparatus is used contains an electrically charged mandrel and an electrically charged screen, the latter having a suitably imprinted pattern for creating a desired printed image. The normal screen or stencil generally comprises a fine mesh element of conductive material having masked non-printing areas. The ink particles are sized to pass through the interstices of the non-masked areas.

The apparatus of the present invention generally includes a hopper containing the desired electroscopic ink. Rotatably mounted in the hopper is a continuously moving agitator for keeping the ink particles in a suspended or levitated state. A distributor roller is disposed beneath the hopper and communicates internally therewith to receive a charge of ink and deposit the same on a continuously rotating belt. The distributor roller in combination with the agitator of the hopper also functions as a metering roller for metering a preselected charge of ink. Provided for operative communication with the surface of the belt is a charging roller with a surface speed different than the surface speed of the belt. The charging roller also aids in providing an even distribution of ink particles transversely across the belt. The belt is driven by a pair of rollers, one of which is charged and serves as the discharge electrode in the electrostatic printing system. The apparatus of the present invention is also provided with a series of suitable adjustments in order to vary dimensions and distances between the moving parts. These adjustments are essential in providing a wide degree of utility to the apparatus so that it is capable of handling a wide variety of types and sizes of ink particles.

Detailed description Referring now in more detail and by reference characters to the drawings which illustrate a preferred embodiment of the present invention, A designates an ink delivery tand charging apparatus which is adapted for use in the electrostatic printing machine described and illustrated in copending application Ser. No. 463,109 filed June 11, 1965. The apparatus A comprises a base plate 1 and adjustably secured to the upper surface thereof near one transverse end is an upstanding bracket 2. The bracket 2 is integrally provided with a base flange 3 for facewise engagement with the upper surface of the plate land an inwardly struck support plate 4 integrally formed at its upper end, substantially as shown in FIGURE 1. Rigidly secured to the upper surface of the support plate 4 by means of bolts 5 is a journal block 6 which retains a pivot shaft 7. Rigidly mounted on the pivot shaft 7 and being pivotal therewith is a lengthwise extending support plate 8. Rigidly secured to the base plate 1 and extending upwardly therefrom at the transverse end opposite the bracket 2 is a bolt 9 which i retained by means of a nut 10. The bolt 9 is designed to engage a clamping plate 11 which is secured to the underside of the support plate 8 by means of bolts 12. The shank of the bolt 9 extends through an elongated slot 13 formed in the plate 8 providing longitudinal adjustment for the support plate 8 and is adapted to retain the support plate 8 in a locked position by means of nuts 14, 1S and washers 16, 17 which engage the upper and lower surfaces of the clamping plate 11. Thus by releasing one of the nuts 14, it is possible to pivot the support plate 8 about the pivot shaft 7 and moreover, it is possible to adjust the relative longitudinal position of the plate 8 with respect to the base plate 1. In this connection, it should be understood that the base flange 3 of the bracket 2 is capable of being fixedly mounted in any of a plurality of selected positions along the base plate 1. Adjustable positioning can be o tained by providing the base plate 1 with a series of vertically extending bolt holes which are longitudinally spaced for securing the bracket 2 in any selected position with suitably sized bolts (not shown) extending through the desired bolt holes.

Adjustably secured to the forwardly presented surface of the support plate 8 is a spacer plate 18 which is secured to spacer blocks 19 by means of bolts 20. Integrally formed with or otherwise rigidly secured to the spacer blocks 19 is an ink hopper 21 which may be molded or formed from any suitable synthetic resin or plastic material. The material selected for the construction of the hopper is not necessarily limited to a plastic or synthetic resinous material but, however, should be formed of a material which is inert with respect to the ink. The bolts 20 extend through an elongated longitudinally extending slot 22 formed within the support plate 8 and are provided with heads 23 which bear against a locking plate 24, the latter being in facewise engagement with the rearward surface of the support plate 8, all as can best be seen in FIGURES 2 and 3. Thus if it is desired to longitudinally shift the position of the hopper 21, it is only necessary to loosen the bolts 20 releasing the frictional engagement between the support plate 8 and locking plate 24. After the hopper 21 has been shifted to the desired longitudinal position, the bolts 20 are tightened so that the heads 23 thereof engage the locking plate 24 which in turn locks the spacer plate 18 and the hopper 21 in a fixed position on the support plate 8.

The hopper 21 is provided with an open upper end 25 sized for accommodation of a removable cover plate 26. The hopper 21 is also provided with an internal chamber 27 which is formed by flat interior front and back walls 28, 29 respectively, and fiat interior side walls 30, the latter of which merge at their lower end into an arcuately shaped bottom wall 31. The bottom wall 31 is provided with an elongated discharge slot 32 which extends the transverse length of the chamber 27, that is to say, it extends the distance between the interior front wall 28 and the interior back wall 29. The hopper 21 is also provided at its lower end with a pair of transversely extending plate receiving slots 33 which open into the discharge slot 32 and are sized to accommodate shiftable closure plates 34, the latter being conventionally shifted by forwardly projecting handles 35 and locked in position by nuts 36. Through the above outlined construction, it is possible to selectively adjust the width of the discharge slot 32 to accommodate electroscopic inks of various properties.

The front and back walls 28, 29 are bored to accommodate bearings 37 and extending transversely through the hopper 21 and journaled in the bearings 37 is an agitator shaft 38 which is provided at its forward end with a pulley 39. The shaft 38 is also axially positioned by means of set collars 40 which engage the interior surfaces of the front and back walls 28, 29 substantially as shown in FIG- URE 3. Secured to the set collars 40 is an agitator 41 consisting of four radially spaced rectangularly shaped agitator blades 41'. By reference to FIGURES 6 and 7. it can be seen that the blades are provided with 2 radiab 1y extending leg portions 42 which are disposed in close proximity to the front and back walls 28, 2'9 and are connected by a bight portion 43 which is sized to closely approach the interior surface of the side walls 30. The agitator blades are circular in cross section and have a relatively small diametral size in the area of approximately inch. The agitator 41 is powered by a conventional variable speed electric motor 44 through a drive belt 45, the motor 44 being mounted on the hopper 21 as shown in FIGURE 1.

Bolted or otherwise rigidly secured to the base of the hopper 21 are a pair of depending axially aligned brackets 46 which are suitably bored for accommodating bearings and journaled therein is a transversely extending roller supporting shaft 47. Keyed or otherwise rigidly secured to the shaft 47 and being rotatable therewith is a distributor roller 48 which is provided with a relatively thick bristlebrush fabric 49 on its annular surface, preferably formed of a mohair pile fabric. The fabric can be secured to the roller 48 by any suitable adhesive. It has also been found that when the fabric 49 is formed of a foamed elastomer material, very effective results have been obtained. The fabric 49 is not necessarily limited to the above materials. The important criteria in selection of the fabric 49 is that the material must be capable of sealing the discharge slot 32 and yet have a sufficient number of cavities or interstices into which the ink powder can be deposited and subsequently transferred to a belt carrier. The fabric 49 must also have a different triboelectric charging potential than the ink selected. The distributor roller 48 is so locatedthat the bristles of the fabric 49 extend upwardly through the discharge slot 32 in the manner as shown in FIGURE 7. Through this construction, the bristles or openings of the distributor roller 48 are able to engage the fine mesh particles of electroscopic ink where the particles of ink will become deposited in the interstices of the brush fabric 49. Also mounted on the undersurface of the hopper 21 is a pair of depending transversely extending cover plates 50 for preventing scattering of the ink particles. At its rearward end, the shaft 47 is provided with a pulley 51 which is driven through a belt 52 by means of a variable speed conventional electric motor 53, the latter being mounted on the hopper 21 as shown in FIG- URE 3.

Bolted or otherwise rigidly secured to the base plate 1 is a journal block 54 and journaled therein is a shaft 55 which carries a drive roller 56. The journal block 54 is adjustably secured to the base plate 1 through bolts which ride in elongated slots. The position of the journal block 54 is adjusted by means of set screws 57 which are retained by a bracket 58, the latter being bolted or otherwise rigidly secured to the base plate 1, substantially as shown in FIGURE 2. One of the set screws 57 is secured to the journal block 54 and serves as a guide. The other two set screws 57 provide adjustment and positioning of the roller 56. Bolted or otherwise rigidly secured to the upper surface of the base plate 1 in a fixed position is a journal block 59 which is longitudinally spaced from the journal block 54. Journaled in block 59 is a shaft 60 which retains a feed roller 61 and trained around the feed roller 61 and the drive roller 56 is an ink transporting belt 62. The belt 62 is also preferably formed of a relatively thick mohair pile fabric which is engaged by the distributor roller 48, in the manner as shown in FIG- URE 2. Thus, as the belt 62 passes by the feed roller 48, the bristles ofthe belt 62 will engage the ink particles retained in the interstices of the fabric layer 49 on the roller 48 and thereby carry the particles away from the roller 48. The rollers 56, 61 are of the same diametral size. The roller 56 is crowned centrally of its axial length and the roller 61 is substantially cylindrical for its axial length. Moreover, the roller 56 may be provided with retaining flanges on each of its transverse ends. By means of the crowned portion on the drive roller 56, the belt is longitudinally retained in alignment with the distributor roller '48. Moreover, this type of construction prevents transverse shifting of the belt 62 during rotation thereof. In order to pervent a convexing of the belt 62, and to maintain a complete transverse belt-to-roller contact with the distributor roller 48, a cylindrical idler roller '63 is disposed beneath the roller 48 and engages the underside of the belt 62. The idler roller 63 is keyed to. a shaft 64 which is journaled in a bearing block 65, the latter being bolted or otherwise rigidly secured to the upper face of the base plate 1.

The drive roller 56 is driven by a conventional variable speed electric motor 66 through a drive belt 67, which is also trained about a pulley '68 mounted on the rearward end of the shaft 55. The motor 66 is, of course, secured to any suitable supporting structure (not shown).

Adjustably secured to the upstanding leg of the bracket 2 by means of bolts 69 which ride in elongated slots 70 is a journal block 71. Vertical positioning of the journal block 71 is provided through three transversely aligned set screws 72, the center set screw 72 providing vertical alignment and the outer set screws 72 providing vertical positioning. Carried by the journal block 71 is a shaft 73 and mounted on the shaft 73 is a charging roller 74, the latter being provided with a pile fabric 75 on its outer surface. Again, the criteria employed for selecting the pile fabric 75 is the same used in the selection of the fabric 49 on the roller 48. By reference to FIGURE 1, it can be seen that the roller 74 is so located that the fabric portion 75 thereof engages the surface of the belt 62 as it passes therebeneath. The surface speed of the roller 74 is different than the surface speed of the belt 62 and this friction efi'ect creates a triboelectric charge on the ink particles retained in the interstices of the belt 62, all in a manner to be more fully described in detail. At its rearward end, the shaft 73 is provided with a pulley 76 and is driven through a drive belt 77, which is in turn connected to a variable speed electric motor 78, the latter being secured to any suitable supporting structure (not shown). In this connection, it should be understood that each of the drive motors 44, 53, 66 and 78 could be replaced by a single electric motor with conventional variable speed drive means operatively connected to each of the rollers 56, 48 and 74 and the agitator 41.

It should be noted that the distributor roller 48 provides uniform initial distribution of ink particles. In addition to triboelectric charging of the ink particles, the charging roller 74 disperses aggregates of ink particles, spreading the ink to give a uniform coating and thereby aids in preventing ghosting effects in the actual printing operation.

An ink retaining tray 79 is provided for use with the apparatus A and is designed to collect the excess ink particles metered from the hopper 21. The tray 79 is pro vided with a base plate 80 and a pair of upstanding side walls 81 which are closed by a transverse end wall 82 and are open at the opposite end. The walls 81 are provided with elongated slots 83 which are designed to accommodate the shafts 55, 60 and 64 as the tray 79 is shifted to the closed position or to the right, reference being made to FIGURE 1, where the tray 79 is disposed immediately beneath each of the rollers. The end wall 82, which is relatively thick in its longitudinal dimension, is provided with an arcuate portion 84 to accommodate the roller 56. The lower portion of the continuous belt 62 moves in close proximity to the base plate 80 and picks up ink particles which fall into the tray79. The positions and relationships of the various rollers with respect to each other is schematically illustrated in FIGURE 10. Also illustrated is a conventional screen S and a conventional mandrel M, the details of construction of which are more fully described in copending patent application Ser. No. 463,109 filed June 11, 1965. The circuitry for creating the electrostatic field between the mandrel M, the screen S and the feed roller 61 is also schematic-ally illustrated in FIGURE 10. By the circuitry illustrated, it can be seen that by use of the double-pole, double-throw switch P, it is possible to reverse the current between the screen S and the feed roller 61.

Operation In use, apparatus A is secured to the electrostatic printing machine described in the aforementioned copending application and is designed to meter a preselected amount of electroscopic ink from the hopper 21 through the distributor roller 48. The ink is deposited upon the belt 62 and is triboelectrically charged by the charging roller 74. The ink is thereafter delivered to the screen S as the belt 62 passes over the feed roller 61. In the area of contact between the belt 62 and the screen S, mechanical and electrostatic forces combine to urge the ink particles to travel through the openings of the screen S to the mandrel M. The element to be printed is, of course, positioned on the mandrel M for each printing operation or printing cycle.

As the belt 62 passes over the feed roller 61, it is always maintained in intimate contact with the screen S so that the fabric on the belt 62 is capable of forcing the ink particles carried on the belt through the apertures of the screen S. After the ink particles are forced through the screen S, they are suspended and moved in the electrostatic field toward the mandrel or counterelectrode M. It is, however, possible to adjust the position of the feed roller 61 with respect to the screen in order to maintain a uniform contact. For this purpose, the base plate 1 is provided with suitable means for shifting the entire position of the apparatus A. It is also desirable to maintain a predetermined amount of tension on the belt 62 for various types of electroscopic inks. In order to provide a greater degree of tension on the belt, the screws 57 are turned to move the journal block 54 in a direction away from the feed roller 61. Conversely, if it is desired to release the tension on the belt 62, the screws 57 are turned in a direction to shift the journal block 54 toward the feed roller 61.

Also of critical importance in proper metering and distribution of electroscopic ink is the degree of contact that the distributing roller 48 has with the belt 62. This degree of contact can be adjusted by pivoting the position of the support plate 8 about the pivot pin 7. By pivoting the support plate 8, the intimacy of contact between the belt 62 and the roller 48 can thereby be adjusted to a desired degree. In like manner, the charging roller 74 can be positioned with respect to the belt 62 by adjustment of the screws 72. It has been found in connection with the present invention that this wide versatility of adjustment is necessary in order to provide a delivery system which is capable of accommodating a wide range of electroscopic inks.

By reference to FIGURE 1, the most optimum results have been found when each of the rollers rotate in the directions indicated by the arrows. In other words, the feed roller 61, the drive roller 56 and the idler roller 63 all rotate in a clockwise direction. The distributor roller 48 and the agitator 41 both rotate in a counterclockwise direction. However, the charging roller 74 rotates in a clockwise direction. The roller 74 is positioned so that it creates a slight frictional contact with the belt 62. In operation, there is a continual interchange of ink between the charging roller 74 and the belt 62, such that the roller 74 adds ink to depleted areas of the belt and picks up ink from areas having an excess thereof. The roller 74 also serves to break up and spread loosely aggregated ink particles.

In this connection, it should be understood that the charging roller 74 and the belt 62 would, under a state of steady metering and delivery of ink particles to the screen S reach a state of dynamic equilibrium. In this state of dynamic equilibrium, both the charging roller 74 and the belt 62 would have a definite and specific quantity of ink particles contained thereon per unit of area. Moreover, in the state of equilibrium there would be a constant interchange of ink between the roller 74 and the belt 62. If the ink were non-uniformly removed from the belt 62 as is the case in the printing operation, this equilibrium state will be upset and ink will =fiow preferentially from the charging roller 74 to the depleted areas of the belt 62, in the event that the depleted areas Were not filled by the distributor roller 48. However, it is to be understood that the belt 62 is generally loaded rather heavily with ink particles so that fraction removed by printing is relatively small and return to equilibrium loading readily occurs through the action of the charging roller 74.

By means of the above outlined construction, even distribution and a continuous supply of ink is provided at the screen S. The bristiles of the roller 4-8 engage the particles of electroscopic ink in the hopper 21 as the bristles pass through the discharge slot 32. The particles of ink are deposited in the interstices of the fabric 49' and are engaged by the bristles of the belt 62 as the fabric 49 engages the belt 62. The ink particles are then picked up by the belt 62 and transported in the direction of the screen where they are brushed through the apertures of the screen S. The distributor roller 48 not only provides a metering function but also provides a function of spreading for maintaining even distribution of the ink charge. As previously indicated, the rate of ink delivery can be adjusted by the positioning of the plates 34.

In actual practice, the plates 34 are not absolutely necessary in that the rate of ink delivery can be regulated by regulating the speed of rotation of the delivery roller 48. However, the shiftable plates 34, in many cases, provide an additional means of regulating the size of the discharge slot 32 and hence the rate of ink delivery. It can be seen that the distributor roller 48 serves as a closure for the discharge slot 32 and is effective because of the reasonably close fit and because of the fact that the ink particles are so fine they do not flow. There is a tangential approach of both the metering roller 48 and the agitator 41 which assures a uniform metering of ink and aids in breaking up a caking of this highly sensitive powder. In connection with the present invention, it is important to have a tangential approach of the agitator 41 and the metering roller 48 if not an absolute tangential contact as the two rotate. It can also be observed that the base of the hopper 21 is arcuate which aids in delivery of the ink particles to the discharge slot 32.

As previously indicated, the screen S is provided with a pattern formed by a solid portion and a screen portion. Whereas part of the ink particles under the influence of the electrical and mechanical forces go through the open interstices of the screen S, others are electrostatically attracted to the closed surfaces of the screen S. These latter particles are removed by periodically reversing the position of the switch P to reverse the electric field between the feed roller 61 and the screen S. -As this occurs, and as the screen is passed through a forward and reversing movement, or complete cycle, by the feed roller 61, the excess portion of ink which has gathered on the screen will be attracted toward and returned to the belt 62. The absolute electrical polarity for printing or for ink removal depends on the relative position in the triboelectric series of ink particles and the pile of the belt fabric. For a given ink-belt combination, this is readily determined by trial or by measurements of triboelectric position with an electrometer. It has been found in connection with the present invention that the belt 62 should be rotated at a speed of four revolutions per second for a screen stroke for 10 inches, that is the distance in which the screen S moves in one cycle. The velocity of the charging roller 74 should be twice the velocity of the belt 62. Moreover, the velocity of the distributor roller should be maintained at approximately 0.02 revolution per second and the velocity of the agitator should be maintained at approximately one revolution per second. As the screen stroke is increased or decreased, the other velocities are changed accordingly.

The ink delivery system of the present invention is particularly adaptable for use in the printing method for printing on curved surfaces and described in copending application Ser. No. 472,829, filed July 19, 1965, and which relates to electrostatic screen process printing. In the printing on curvilinear surfaces such as conically shaped cups, the cup is positioned in an axis of rotation so that the exterior wall tangentially approaches and departs from the screen. Thus, the printing will occur along an elemental line of closest approach between the cup and screen. The roller 61 of the delivery system A also tangentially approaches and departs from the screen providing selected quantities of ink to the screen at the desired line of tangency. The substrate or container is rotated at approximately the same rate of speed of the movement or rotation of the screen so that a continuing line of tangency occurs between the surface of the container and the surface of the screen. Simultaneously with the rotation of the screen and the container, electroscopic ink particles are moved toward and through the screen to the substrate by the electrostatic field. The ink particles are passed through the screen along this line or band of tangency. In this manner, it is possible to provide electrostatically printed images on the surface of a curvilinearly shaped article, such as a conically shaped container.

It should be understood that changes and modifications in the form, construction, arrangement and combination of parts presently described and pointed out may be made and substituted for those herein shown without departing from the nature and principle of our invention.

Having thus described our invention, what we desire to claim and secure by Letters Patent is:

1. The method of metering and delivering electroscopic ink to the print receiving element of an electrostatic printing system, said method comprising accumulating a quantity of electroscopic ink in an ink housing, constantly agitating said ink particles to maintain the same in a suspended state, engaging ink in said housing by the pile fabric surface of a metering roller extending through a slot in said housing, rotating an agitator in said housing and said metering roller in pretimed relationship, regulating the speed of said agitator and said metering roller to control the amount of ink accumulation by said metering roller, transferring the ink from the pile fabric surface of said roller to the pile fabric surface of a delivery belt, and transporting said ink to an electrostatic field.

'2. The method of claim 1 further characterized in that the particles carried on said delivery belt are triboelectrically charged before they enter the electrostatic field.

3. The method of claim 1 further characterized in that a charging roller is rotated in a direction opposite to the movement of the delivery belt and engages the ink particles on said belt for imparting a triboelectric charge to the ink particles.

4. The method of claim 1 further characterized in that intimacy of contact is maintained between said belt and metering roller so that the ink particles from the roller are transferred to the interstices of the pile fabric surface on said belt. I

5. The method of metering and delivering electroscopic ink to the print receiving element of an electro-' static printing system, which includes a movable printing screen, said method comprising accumulating a quantity of electroscopic ink in an ink housing, constantly agitating said ink particles to maintain the same in a suspended state, engaging ink in said housing by the pile fabric surface of a metering roller extending through a slot in said housing, rotating an agitator in said housing and said metering roller in pretimed relationship, regulating the speed of said agitator and said metering roller to control the amount of ink accumulation by said metering roller, transferring the ink from the pile fabric surface of said roller to the pile fabric surface of a delivery belt, transporting said ink to an electrostatic field. curving said delivery belt about a roller so that the belt approaches aparallel relationship with the screen in close proximity to the screen, and maintaining a tangential approach and departure relationship between the pile fabric surface of the belt and the movable screen.

6. The method of claim 5 further characterized in that the ink is transferred from the belt at a tangential line of closest approach existing between the screen and the surface of the delivery belt.

7. The method of claim '5 further characterized in that the potential difference existing between the delivery belt and the screen is reversed after a quantity of ink has accumulated on said screen.

References Cited UNITED STATES PATENTS 2,318,570 5/1943 Carlton. 2,832,977 5/1958 Walkup et al 15-15 3,099,856 8/1963 'Eichorn et al. 15-l.5 XR 3,149,356 9/1964 Murray et al 151.5 3,152,012 10/ 1964 Schatfert. 3,296,965 1/1967 Reif et al. 101-170 3,306,193 2/ 1967 Rarey et al 101--114 EDGAR S. BURR, Primary Examiner.

US. 01. int. 

